Surface Mass Spectrometry

Emission of atoms and molecules from surfaces due to ion irradiation: Emission characteristics; surface ionization processes; compositional changes in multicomponent systems. The following graph depicts the dependence of the ionization probability on the surface work function. The exponential correlation agrees with the electron-tunneling model of ion formation.

Intensity as a function of work function change DF for C^- and Si^- secondary ions sputtered from graphite and silicon, respectively. The change of DF was effected by varying amounts of Cs in the near-surface region.

The detection of small doubly-charged molecular anions by means of highly sensitive mass spectrometry has been studied. The production of these gas-phase dianions is accomplished by sputtering the specimen with Cs⁺ ions with an energy of a few keV. It is demonstrated that dianions can be detected most easily when the molecular ion has an odd total mass; then, the dianions will show up at half-integral mass numbers in the mass spectrum. The flight time through the mass spectrometer of several µs establishes a lower limit with respect to the intrinsic lifetimes of these dianions.

Mass spectrum in the range of the doubly charged C₁₀²^- cluster ion obtained by 14.5-keV Cs⁺ ion bombardment of a graphite specimen. A mass resolution of M/DM_0.1~ 1400 was used to record this spectrum.

The existence of long-lived (metastable) molecular anions such as N₂^-, CO^-, H₂^-, D₂^-, H₃^-, and D₃^- was demonstrated. These anion species were produced by sputtering of appropriate targets with Cs⁺ ions and were identified by high-sensitivity secondary-ion mass spectrometry. The figures below show exemplary mass spectra from these analyses.

	High-resolution mass spectrum around 28 amu obtained by 14.5-keV Cs⁺ sputtering of a TiAlN(C,O) specimen. The arrows at the ²⁸Si-peak mark the width of the 10%-intensity level; the resulting value DM_0.1 = 0.00217 amu yields a mass resolution M/DM_0.1 = 13000.
	Mass spectra in the mass region m/q ~ 2 amu obtained by 14.5-keV Cs⁺ sputtering of HfH₂. The H₂^- peak is separated from that of D^- using high mass resolution (M/DM ~ 4000).

From the SIMS intensities, the following relative abundances were derived for the molecular hydrogen anions: H₂^-/H^- = 3.5´10^-⁶, H₃^-/H^- = 1.7´10^-⁸, D₂^-/D^- = 3.1´10^-⁵, D₃^-/D^- = 7´10^-⁹.

· Structural and electronic modifications of single crystal surfaces of semiconductors (e.g., Si, Ge, GaAs, InP) by low-energy ion bombardment. The diagrams show the ion-bombardment induced amorphization fluences for two semiconductor surfaces.


Amorphization fluence of Ge(100) as a function of Ar⁺ ion energy.	Amorphization fluence of GaAs(110) as a function of Ar⁺ ion energy. The parameter is the ion flux density.

· Micro-area ion implantation in various materials with focused ion beams.